Anaerobic Bacterial Metabolism: Definition & Process

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  • 0:08 Anaerobic bacterial metabolism
  • 1:27 Basic metabolic respiration
  • 3:34 Anaerobic respiration
  • 4:20 Nitrate respiration
  • 5:27 Sulfate respiration
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Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Bacteria are metabolically versatile and can grow in a range of environments. Many bacteria grow in environments without oxygen using anaerobic respiration and fermentation. This lesson will discuss the process of anaerobic respiration in bacteria.

Anaerobic Bacterial Metabolism

We live on a microbial planet. It is estimated there are at least 10^30 bacterial cells on the planet. To put that into perspective, that is more than all the predicted stars in the universe. There is also a microbial world living inside of you. There are trillions of bacterial cells living in association with your body, outnumbering human cells by 10 times.

Bacteria are able to pull off this amazing feat of world domination by having metabolic abilities that allow them to survive in some pretty crazy environments. As mammals, we associate life with nutrients, food, and oxygen. Lacking any one of these components results in death. Bacteria, on the other hand, still require nutrients and food sources, but many don't require oxygen at all. This allows them to survive in deep ocean vents, inside your anaerobic gut, and deep within the Earth.

So how exactly do these cells survive and grow in these environments?

The answer is by using processes like anaerobic respiration, which is breathing or respiring something besides oxygen, and fermentation. In this lesson, we will discuss the details of anaerobic respiration.

Basic Metabolic Model

Let's start with the most basic metabolic model of a bacterial cell using glucose as an energy source and respiring oxygen. This is the process of aerobic respiration, or the process in which a compound is oxidized using oxygen as the terminal electron acceptor and resulting in a proton motive force.

First, the cell will use glycolysis to strip glucose of its electrons, breaking it down into pyruvate and capturing the electrons on the electron carrier NADH. The pyruvate then enters the citric acid cycle, where it is oxidized all the way to carbon dioxide. Like during glycolysis, the electrons stripped from pyruvate are stored as NADH. The NADH then enters the electron transport chain and donates its electrons to a series of proteins, enzymes, and molecules that pass along the electrons. The energy of electron transfer is used to pump protons out of the cell. At the end of the chain, the electrons are dumped onto oxygen, generating water and ending the cycle.

The proton-pumping action of the electron transport chain results in a proton motive force, or an energized state across a membrane resulting from a proton gradient. This means that the energy of the proton gradient, high on the outside of the cell and low on the inside, can be exploited. During respiration, this proton motive force is used by the ATP synthase to make ATP, the vital high-energy molecule that supports growth and synthesis of all the major cellular compounds.

The ATP synthase opens a channel through the membrane and, as the protons flow through the channel down the gradient, the energy turns the ATP synthase, resulting in a torque force that is used to add a phosphate group to ADP to generate ATP. This process of using an electron transport chain to generate a proton motive force used for ATP synthesis is referred to as oxidative phosphorylation.

Anaerobic Respiration

Why did we spend so much time talking about aerobic respiration if we are interested in anaerobic respiration? Well, the basic steps in the process are the same, so by understanding aerobic respiration we can better understand anaerobic respiration, which is simply the process in which a compound is oxidized using something besides oxygen as the terminal electron acceptor and resulting in a proton motive force.

In both aerobic and anaerobic respiration, the end goal is the same: oxidative phosphorylation - or, in simple terms, to generate a proton motive force that can be used to make ATP using the ATP synthase. So now we can tweak the aerobic respiration model to understand how anaerobic respiration works.

Nitrate Respiration

Let's look at two examples:

Soil environments are characterized by anaerobic zones, and many contain sources of nitrogen in the form of nitrate. In this type of environment, many bacteria grow using the process of anaerobic respiration with nitrate as an electron acceptor.

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